THE PROCESS IN MOTION

Why do hardness and alkalinity not do the same job?

Start the scene or choose a stage. The animation simplifies the mechanism; what follows gives the details and the available references.

▶ Play the process01Two counts in the same water02Alkalinity, in the cup03Hardness, a less direct effect04In the kettle, the pair makes scale

Two counts in the same water

The same water carries two kinds of ions, which are counted separately. Total hardness is the concentration of cations, calcium and magnesium in practice. Total alkalinity is the water's capacity to resist changes in pH, and it is measured by its concentration of bicarbonate ions, the ones that make the buffer. The word misleads twice: alkalinity is not pH, and an “alkaline” pH refers to another concept.

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1. Two counts in the same waterThe same water carries two kinds of ions, which are counted separately. Total hardness is the concentration of cations, calcium and magnesium in practice. Total alkalinity is the water's capacity to resist changes in pH, and it is measured by its concentration of bicarbonate ions, the ones that make the buffer. The word misleads twice: alkalinity is not pH, and an “alkaline” pH refers to another concept.
2. Alkalinity, in the cupContaminants aside, this is probably the parameter that weighs most on taste. Many desirable coffee compounds are acids, and a high alkalinity modifies them chemically as they are extracted. In Zurich, the least alkaline water tested, 20 ppm, was preferred, perhaps on a coffee darker than the specialty average; the community uses 40 to 50 ppm, and Gagné finds 20 ppm a little sour on light roasts.
3. Hardness, a less direct effectProfessional consensus holds that harder water favors the extraction of certain compounds, though the mechanism has not been demonstrated in coffee: the hypothesis is that these ions speed up their diffusion. Magnesium would tend to extract slightly more caffeine and less quinic acid than calcium. Other trials measured no difference in extraction yield, and see them rather as flavor modulators.
4. In the kettle, the pair makes scaleTemporary hardness and permanent hardness are used mostly to talk about scale. It forms when cations recombine with bicarbonate into calcium carbonate, which precipitates and damages machines. Temporary hardness, called carbonate hardness, is the smaller of the two quantities, generally the alkalinity: it is the maximum possible scale. Permanent hardness is what remains of the cations once all the bicarbonate is consumed.

To remember: Hardness counts the cations, alkalinity counts the buffer: the second weighs most on taste, and it is the smaller of the two that sets the possible scale.

Read the full article and its sources ↓

Total alkalinity is the water's capacity to resist changes in pH. In practice, it is measured by its concentration of bicarbonate ions, since they are what makes the buffer just discussed.

Beware of the word, which misleads twice. Alkalinity is not pH. And an “alkaline” pH refers to another concept, despite the kinship of the names.

Contaminants aside, this is probably the parameter that weighs most on taste. Why this one? Because many of the desirable compounds in coffee are acids. A high alkalinity modifies them chemically as they are extracted, even before they reach the cup.

Now the figures. In the sensory trials of the Zurich laboratory, the least alkaline water tested, 20 ppm, was the one preferred. This result needs a caveat: the coffee used may have been roasted darker than the specialty average. A low alkalinity would then have been favored, because it preserved an acidity that was otherwise discreet.

The community, for its part, commonly uses 40 to 50 ppm. Gagné finds 20 ppm a little too sour on light roasts. The right figure therefore depends on the coffee that goes into it, not only on the water.

Gagné 2021, pp. 23-24; Wellinger and Yeretzian 2015

Total hardness is the concentration of cations, calcium and magnesium in practice. Its effect is less direct: the consensus among professionals is that harder water favors the extraction of certain compounds, though the mechanism has not been demonstrated in coffee; the hypothesis, based on gas chromatography data, is that these ions increase the rate of diffusion of specific compounds.

Magnesium would tend to extract slightly more caffeine and less quinic acid than calcium; sodium would favor a very different profile, several acids favored, caffeine and eugenol disfavored. Other preliminary experiments failed to measure any difference in extraction yield when varying hardness, and suppose that calcium and magnesium act as flavor modulators even at concentrations imperceptible in the water.

Field anecdote has it that a higher calcium hardness favors body and sweetness, and that a little magnesium adds complexity. Colonna-Dashwood's rough rule: softer water, with low alkalinity and hardness, extracts less and gives a brighter acidity, imitating certain aspects of a lighter roast.

Gagné 2021, pp. 24-25, after Colonna-Dashwood and Hendon, Water for Coffee (2015), Hendon, Colonna-Dashwood and Colonna-Dashwood, Journal of Agricultural and Food Chemistry 62 (2014), Smrke (pers. comm.) and Colonna-Dashwood, January 5, 2016

Temporary hardness and permanent hardness are used mostly to talk about scale. Scale forms when cations recombine with bicarbonate into calcium carbonate that precipitates and damages machines. Temporary hardness, called carbonate hardness, measures the maximum amount of scale the water can form: it is the smaller of the two quantities, alkalinity or hardness, in equivalent units, generally the alkalinity.

Titration kits sold for alkalinity are often, wrongly, called carbonate hardness tests. Permanent hardness is what remains of the cations once all the bicarbonate is consumed, that is, the difference between the two, zero when alkalinity exceeds hardness.

Gagné 2021, pp. 25-26